1. Field of the Invention
This invention relates to a braking apparatus for a vehicle. In particular, this invention relates to an braking apparatus for a motorcycle which employs an electronic braking system.
2. Description of the Background Art
It is well known to provide hydraulic braking devices for vehicles with an antilock brake system (hereafter to be referred to as the “ABS”) for suppressing slippage of a wheel under specified conditions. Some hydraulic type vehicle braking devices are capable of electrically controlling the hydraulic pressure applied to a brake caliper by means of a hydraulic modulator. Such a braking apparatus is disclosed, for example, in Japanese published patent document JP 2890215.
In the vehicle braking apparatus described above, when unreasonable amount of slippage is detected at a wheel, the ABS is activated to reduce the hydraulic pressure applied to the brake caliper to a predetermined value, so as to suppress the slippage of the wheel. However, according to this system, there is a slight time lag immediately after the activation of the ABS, even though the time lag is practically negligible.
Some motorcycle braking devices employ an electronic brake system, also known as a so-called brake-by-wire system, which is designed to electrically detect an amount of operation of a brake actuating unit such as a brake lever, and operate a wheel braking mechanism by hydraulic pressure generated by a hydraulic modulator, based on the detected amount of braking operation. Such a braking apparatus is disclosed, for example, in Japanese published patent document JP-A 310717/2001.
In some motorcycles employing the brake-by-wire system, for example, a main brake path having a normally-open electromagnetic on-off valve is connected to a master cylinder. The master cylinder works in conjunction with a brake lever, and is connected to a brake caliper via the main brake path. The brake caliper serves as a wheel driving mechanism and applies a braking force to the wheels by means of a hydraulic pressure operation. A hydraulic modulator, which supplies the brake caliper with hydraulic pressure generated by the electric actuator, is connected to a point in the main brake path closer to the brake caliper relative to the electromagnetic on-off valve.
In this type of braking apparatus, the electromagnetic on-off valve is switched to the closed state only during braking operation by energization of a solenoid, whereby the main brake path is closed. The main brake path is thus separated into the master cylinder side and the brake caliper side, so that a hydraulic pressure corresponding to the hydraulic pressure generated by the brake lever is applied to the brake caliper. As a result, the vehicle is braked.
In the conventional electronic braking apparatus for a motorcycle as described above, the solenoid of each of the electromagnetic on-off valves used in the braking apparatus is required to be energized for performing on/off control of the electromagnetic on-off valve. This poses a problem in that the power consumption of the brake system is increased, and hence, the burden to the battery is increased.
However, if, for the purpose of suppressing the power consumption, the vehicle is configured so that the electromagnetic on-off valve as described above is opened in the non-energized state while the vehicle is at a halt, the electromagnetic on-off valve, which has been closed when the brake is operated, must be switched over to the open state every time the vehicle is stopped by the brake operation. Therefore, if the vehicle is stopped when there is pressure difference between the side of the main brake path closer to the brake caliper relative to the electromagnetic on-off valve and the side of the main brake path closer to the master cylinder relative to the electromagnetic on-off valve, a problem is posed. Specifically, the difference in hydraulic pressure will cause stroke variations in the brake actuating unit when the electromagnetic on-off valve is switched over. This deteriorates the performance of the brake device.
Therefore, an object of this invention is to provide a vehicle braking apparatus which is capable of detecting wheel slippage, and activating the ABS so as to reduce the time lag after the activation of the ABS.
It is a further object of this invention to provide a brake control apparatus for a motorcycle which is capable of performing a brake operation to stop the vehicle without causing stroke variations in a brake actuating unit, and which improves brake device performance.